How is the QTc (QT corrected) interval calculated in a patient with a 12-lead electrocardiogram (ECG)?

How QTc is Calculated

The QTc (corrected QT interval) is calculated by measuring the QT interval from the beginning of the QRS complex to the end of the T wave, then dividing this value by a function of the RR interval to adjust for heart rate—most commonly using Bazett's formula (QT/√RR) in clinical practice, though Fridericia's formula (QT/∛RR) is more accurate and increasingly recommended. 1

Measuring the QT Interval

Measurement technique:

• Measure from the onset of the QRS complex to the end of the T wave 1
• Select the lead with the most well-defined T wave end, typically leads II, V3, or V5 2, 3
• The longest QT value across leads should be used 1, 3
• Identify the T wave end by drawing a tangent to the steepest downslope of the T wave and marking where it intersects the isoelectric baseline 1, 2

Critical measurement considerations:

• Use the same lead for serial measurements in the same patient to ensure consistency 1, 2
• When the P wave is superimposed on the T wave (common in infants or tachycardia), extrapolate the T wave end using the tangent method 1
• Always visually validate automated computer measurements, as they are often inaccurate 1, 2

Correction Formulas

Bazett's formula (most commonly used):

• QTc = QT (in seconds) / √RR interval (in seconds) 1
• This remains the standard in clinical practice despite known limitations 1
• Major limitation: Overcorrects at heart rates >90 bpm and undercorrects at heart rates <50 bpm 1, 2, 4

Fridericia's formula (more accurate):

• QTc = QT (in seconds) / ∛RR interval (in seconds) 1, 3
• Performs better than Bazett's, especially at higher heart rates 3, 4, 5
• Recommended by the FDA and increasingly by major cardiology societies 3, 4
• Recent data shows Fridericia reduces false-positive prolonged QTc reports by 21% compared to Bazett's 6

Linear regression formulas:

• Framingham formula: QTc = QT + 0.154(1-RR) 7, 5
• Hodges formula shows the least correlation with heart rate (correlation coefficient 0.11 vs 0.33 for Bazett's) 5
• The American College of Cardiology recommends linear regression functions over Bazett's formula for optimal accuracy 1, 2

When NOT to Calculate QTc

Avoid QT correction in these situations:

• Large RR interval variability (e.g., atrial fibrillation) 1, 3
• Heart rate <40 bpm or >120 bpm—correction formulas are unreliable at extremes 2, 3
• Unreliable identification of T wave end 1
• New bundle branch block or QRS >120 ms (requires adjustment for QRS duration) 1, 3

Practical Clinical Application

For extreme heart rates:

• If HR <50 bpm: have patient perform mild aerobic activity to increase HR closer to 60 bpm before measuring 2
• If HR >90 bpm: allow additional rest time to achieve lower HR, or preferentially use Fridericia's formula 2, 3

Normal values:

• Men: QTc <450 ms 2, 3, 8
• Women: QTc <460 ms 2, 3
• Values >500 ms or increase >60 ms from baseline indicate high risk for torsades de pointes 1, 3, 8

Common Pitfalls to Avoid

• Relying solely on automated measurements without visual confirmation—computers frequently miscalculate the T wave end 1, 2
• Using Bazett's formula at heart rates significantly above or below 60 bpm—this produces substantial errors 1, 2, 4
• Measuring different leads for serial comparisons—this introduces variability unrelated to true QT changes 1, 2
• Failing to account for gender differences—women have 8-10 ms longer QTc than men 1, 3
• Not documenting which correction formula was used—this makes serial comparisons unreliable 1